1. Field of the Invention
This invention is directed to the catalytic cracking of petroleum fractions, and specifically to an improved process and an apparatus for increasing gasoline octane number and reducing the pour point of light distillate oils. More specifically, this invention is directed to a process for improving the octane number and pour point of gasoline and light distillate oil, respectively, which are produced in a Fluid Catalytic Cracking unit.
2. Description of the Prior Art
Hydrocarbon conversion processes utilizing crystalline zeolites are well known in the art. Crystalline zeolites have been found to be particularly effective for a wide variety of hydrocarbon conversion processes, including the catalytic cracking of a gas oil to produce motor fuels, and having been described and claimed in many patents, including U.S. Pat. Nos. 3,140,249; 3,140,251, 3,140,252; 3,140,253; and 3,271,418. It is also known in the prior art to incorporate the crystalline zeolite into a matrix for catalytic cracking, and such disclosure appears in one or more of the above-identified U.S. patents.
A dominant process in the field of catalytic cracking using crystalline zeolites is Fluid Catalytic Cracking (hereinafter FCC). This process is so named because the catalyst is in the form of spray-dried microspheres having an average size of 60 microns in diameters. When suspended in an oil vapor or gas, the microspheres act like a fluid.
An FCC reactor typically comprises a thermally balanced apparatus assembly which includes a reactor vessel containing a mixture of regenerated catalyst and the feed, and a regenerator vessel wherein spent catalyst is regenerated. The feed is converted in the reactor vessel over the catalyst, and coke simultaneously forms on the catalyst, thereby deactivating the same. The deactivated (spent) catalyst is removed from the reactor vessel and conducted to the regenerator vessel, wherein coke is burned off the catalyst with air, thereby regenerating the catalyst. The regenerated catalyst is then recycled to the reactor vessel. The reactor-regenerator assembly must be maintained in steady state heat balance so that the heat generated by burning the coke provides sufficient thermal energy for catalytic cracking in the reactor vessel. The steady state heat balance is usually achieved and maintained in FCC reactors by controlling the rate of flow of the regenerated catalyst from the regenerator to the reactor by means of an adjustable slide valve in the regenerator-to-reactor conduit.
The product stream or effluent of the catalytic cracker is usually fractionated into a series of products, including: gas, normally conducted to a gas treatment plant; gasoline; light cycle gas oil; and heavy cycle gas oil. A portion of the heavy cycle gas oil is usually recycled into the reactor vessel and mixed with fresh feed. The bottom effluent of the fractionator is generally allowed to settle. The solids-rich portions of the settled product is also recycled to the reactor vessel in admixture with the heavy cycle gas oil and feed.
In a modern version of the FCC reactor, the regenerated catalyst is introduced into the base of a riser reactor column in the reactor vessel. A primary purpose of the riser reactor is to crack the petroleum feed. The regenerated hot catalyst is admixed in the bottom of the riser reactor with a stream of fresh feed and recycled petroleum fractions, and the mixture is forced upwardly through the riser reactor. During the upward passage of the catalyst and of the petroleum fractions, the petroleum is cracked, and coke is simultaneously deposited on the catalyst. The coked catalyst and the cracked petroleum components are passed upwardly out of the riser and through a solid-gas separation system, e.g., a series of cyclones, at the top of the reactor vessel. The cracked petroleum fraction is conducted to a product separation unit, while the coked catalyst, after steam stripping, passes into the regenerator vessel and is regenerated therein, as discussed above. Most of the craking reactions in such modern FCC units take place in the riser reactor. Accordingly, the remainder of the reactor vessel is used primarily to separate entrained catalyst particles from the petroleum fractions.
Further details of FCC processes may be found in U.S. Pat. Nos. 4,309,279 and 4,309,280, as well as in Considine, Douglas M., Editor-in-Chief, Energy Technology Handbook, McGraw Hill Book Co., 1977, pp. 3-231 to 3-233, to which reference is made for a description of the FCC process.
It known that improved results will be obtained with regard to the catalytic cracking of gas oils if a crystalline zeolite having a pore size of less than 7 Angstrom units is admixed with a crystalline zeolite having a pore size greater than 8 Angstrom units, either with or without a matrix. A disclosure of this type is found in U.S. Pat. No. 3,769,202. Although the incorporation of a crystalline zeolite having a pore size of less than 7 Angstrom units into a catalyst comprising a larger pore size crystalline zeolite (pore size greater than 8 Angstrom units) has indeed been very effective with respect to raising of the octane number, nevertheless it did so at the expense of the yield of gasoline.
Improved octane number with some loss in gasoline yield was shown in U.S. Pat. No. 3,758,403. In this patent, the cracking catalyst included a large pore size crystalline zeolite (pore size greater than 7 Angstrom units) in admixture with a ZSM-5 type zeolite, wherein the ratio of the ZSM-5 type zeolite to the large pore crystalline zeolite was in the range of 1:10 to 3:1.
The use of a ZSM-5 type zeolite in conjunction with a zeolite cracking catalyst of the X or Y faujasite variety is described in U.S. Pat. Nos. 3,894,931; 3,894,933; and 3,894,934. The first two patents disclose the use of a ZSM-5 type zeolite in amounts up to and about 5 to 10 wt %; the third patent discloses the weight ratio of ZSM-5 type zeolite to large pore size crystalline zeolite in the range of 1:10 to 3:1.
The ZSM-5 type catalyst, especially virgin catalyst, has exceedingly high activity. Researchers have attempted to take advantage of the activity of fresh ZSM-5 catalyst by adding only small amounts of it to the FCC catalyst. Typical of such work is U.S. Pat. No. 4,309,280, the entire contents of which is incorporated herein by reference. This patent teaches that adding as little as 0.25 wt % ZSM-5 powder to the circulating catalyst inventory in an FCC unit would increase dry gas production by 50% (from 3.9 wt % dry gas to 6.0; see Example 6 in Table 2).
The criticality of using only minuscule amounts of a ZSM-5 type zeolite to achieve improved results with respect to octane number and overall yield has been shown in U.S. Pat. No. 4,368,114. In this patent, the minuscule quantities of the additive catalyst was shown to give the same beneficial results that were once thought obtainable only by adding much larger quantities of ZSM-5 class catalyst.
While the prior art has shown that zeolites, and in particular ZSM-5, can increase gasoline octane in cracking units, its use will be restricted if a significant gasoline yield penalty and high gas make accompany the octane gain. In this situation, only refiners who have available gas handling and lower hydrocarbon upgrading capacities will find zeolites, such as ZSM-5, attractive in their cracking operations.
Another method of increasing octane number is to raise the cracker reactor temperature. This method, however, is very limited, since many units are now operating at maximum temperatures due to metallurgical limitations. Raising the cracker reactor temperature also results in increased requirements for the gas plant (i.e., gas compressor and separator). Since most gas plants are now operating at maximum capacity, any increased load could not be tolerated by the present equipment. Further still, operating an FCC unit at high severity conditions leads to unselective cracking of many feed components.
It can be appreciated from the foregoing that an alternative method which promotes octane number increase in gasoline and pour point reduction in light fuel or distillate oils would be desirable.